1. Field of the Invention
The present invention relates to a device for the generation of analog signals by means of analog-digital converters. It can be applied especially to the field of the direct digital synthesis. More generally, it can be applied to the generation of analog signals starting from digital-analog converters where, for example, it is necessary to reduce the effect of nonlinearity of these converters.
2. Description of the Prior Art
Direct digital synthesis is a technique of frequency synthesis wherein the samples of the sinusoidal signal to be generated are prepared in digital mode and these samples are converted into analog form by means of a digital-analog converter, hereinafter called a DAC. The frequency synthesizers obtained by this technique are highly attractive in terms of volume, weight and energy consumption for they can benefit from large-scale integration. Their other advantages are in particular very high resolution and very small switching times.
One drawback of direct digital synthesizers according to the prior art is that the spectrum of the output signal shows many parasitic lines. Indeed, at the input of the DAC of the synthesizer, the amplitude of the signal is encoded on a number of bits M which is generally smaller than the number of bits N of the digital signal prepared. The passage from N bits to M bits generates an error of quantification or truncation that results in the presence of parasitic lines on the output signal.
In addition, because of the above-mentioned quantification, the transfer function of the DAC, i.e. the output voltage as a function of the input digital words, is a stepped function. These non-linearities then designate the fact then that the steps are not of equal heights. These non-linearities are added to the above mentioned errors of quantification, and are thus defects of the transfer function as compared with an ideal transfer function where the steps would have the same height. The non-linearities of the DAC result in the generation of harmonic frequencies which are aliased owing to the sampling. In other words, parasitic lines are thus created, a parasitic line being a spectral component located at frequencies different from that of the fundamental component of the signal to be synthesized.
According to a known method for eliminating the amplitude quantification and reducing the non-linearities of a DAC, a high amplitude random noise is added to the prepared digital signal, present at the input of the DAC. This random noise is then removed in an analog manner by a second DAC. This method is described for example in the European patent application EP 0452031. The disadvantage of this method is that it lowers the performance characteristics of the phase noise of a direct digital synthesizer especially because of the injection of noise and the difficulty in matching the two DACs.